Alzheimer's disease (AD; also known as senile dementia of the Alzheimer type (SDAT), primary degenerative dementia of the Alzheimer's type (PDDAT), or Alzheimer's) is the most common form of dementia. Most often, Alzheimer's disease is diagnosed in people over 65 years of age, although the less-prevalent early-onset Alzheimer's can occur much earlier. In 2006, there were 26.6 million sufferers worldwide. Alzheimer's is predicted to affect 1 in 85 people globally by 2050.
Alzheimer's disease is a neurodegenerative disease characterised by the presence of senile plaques and neurofibrillary tangles in the brain. The degree of dementia at death correlates better with neurofibrillary tangle numbers than with senile plaques counts. The presence of neurofibrillary tangles in neurons results in the death of those neurons, implying that prevention of tangle formation is an important therapeutic goal. The principal protein that forms the neurofibrillary tangle is the microtubule-associated protein, tau, which assembles into filaments that have the appearance of twisting about each other in pairs and are referred to as paired helical filaments (PHF). PHF are present in different locations in degenerating neurons in the Alzheimer brain and when many aggregate in the neuronal cell body, they produce the neurofibrillary tangle (Lee et al, 2001).
Intraneuronal deposits of tau in the form of typical neurofibrillary tangles of AD or other morphologically distinct tau aggregates in a number of other neurodegenerative diseases, is the basis for grouping these conditions as tauopathies. Thus, in addition to AD, the main examples of the tauopathies are frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17), progressive supranuclear palsy (PSP), Pick's disease, corticobasal degeneration, and multisystem atrophy (MSA). The intracellular tau deposits (usually neuronal but can also be glial) are all filamentous and mostly in a hyperphosphorylated state compared to the level of phosphorylation of tau from control human brain. In the case of AD, this hyperphosphorylated tau is often referred to as PHF-tau because it is derived from the PHF.
Tau is a phosphoprotein, the function of phosphorylation remaining to be unequivocally established. However, increased phosphorylation of tau on multiple serine and threonine residues reduces the ability of tau to promote microtubule assembly and to stabilise assembled microtubules, effects that have been demonstrated both in vitro and in cells. Many studies have shown that PHF-tau from AD brain is more heavily phosphorylated on serine and threonine than tau from control brain. This has been demonstrated partly by protein sequencing and partly by demonstrating that certain monoclonal antibodies only label either PHF-tau or non-phosphorylated tau and not PHF-tau; the epitopes for many of these antibodies have been mapped to particular phosphorylated residues present in PHF-tau and absent from control brain tau. The pathological tau from most other cases of other tauopathies seems to be similarly hyperphosphorylated to PHF-tau.
These findings strongly imply that similar abnormalities in regulating phosphorylation of tau are shared by all the tauopathies including AD.
A number of proline-directed and non-proline directed protein kinases have been suggested to have a role in the generation of PHF-tau in Alzheimer brain, including casein kinase 1. Mammalian casein kinase-1 exists as multiple isoforms CK1α, CK1β, CK1y1, CK1y2, CK1y3, CK1δ and CK1ε. The role of CK1δ as a potential tau kinase is of particular interest since it has been reported that CK1δ protein is increased more than 30-fold in the hippocampus of Alzheimer brain compared to equivalent controls (Ghoshal, N. et al (1999) Am. J. Pathol 155, 1163-1172) while its mRNA content is increased 24-fold (Yasojima, K. et al (2000) Brain Res 865, 116-120) and CK1 has also been shown to be tightly associated with PHF (Kuret, J. et al (1997) J. Neurochem 69, 2506-2515). CK1δ has also been reported to phosphorylate tau at two epitopes detecting using phospho-specific monoclonal antibodies to tau, and exogenous expression of CK1δ in non-neuronal cells reduces binding of tau to microtubules (Li, G. et al (2004) J. Biol. Chem. 279, 15938-15945). Of note in the context of Alzheimer's disease is a report that CK1 activity is stimulated by amyloid beta-peptide (Aβ), a component of the senile neuritic plaques that, together with tangles, characterise Alzheimer brain (Chauhan, A. et al (1993) Brain Res. 629, 47-52). Additional evidence for possible involvement of CK1 in Alzheimer's disease comes from the reported influence of CK1 in the regulation of Aβ production in neurons (Flajolet, M. et al (2007) PNAS USA 104, 4159-4164). Further work has confirmed that at least 6 newly identified phosphorylation sites in PHF-tau (all on serine or threonine residues) can be generated by CK1δ. The finding that a number of phosphorylation sites in PHF-tau for which CK1 is a strong candidate kinase, including three for which it is the only known kinase, implies that CK1 may make an important contribution to the pathogenesis of Alzheimer's disease (Hanger et al (2007) J. Biol. Chem. 282, 23645-23654).
There is therefore a need for CK1δ inhibitors which may be of potential therapeutic benefit in the treatment of neurodegenerative diseases, such as tauopathies including Alzheimer's disease, frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17), progressive supranuclear palsy (PSP), Pick's disease, corticobasal degeneration, and multisystem atrophy (MSA).